1. Field of the Invention
The present invention relates to a buried type semiconductor laser, and more particularly, to a buried type semiconductor laser having a mesa stripe structure with reduced leakage current.
2. Background Art
With the expansion of an optical fiber communication network, there is a growing demand for a semiconductor laser capable of high-speed, high-temperature operation in recent years. As such a semiconductor laser, a buried type semiconductor laser having such a structure that a mesa stripe-shaped light guide is formed on a semiconductor substrate and current block layers are buried on both sides thereof is being widely used (for example, see Japanese Patent Laid-Open No. 4-320083).
FIG. 17 shows a cross-sectional structure of the above described conventional buried type semiconductor laser. This semiconductor laser 1 is made of an n-type InP substrate 2. On the InP substrate 2, an n-type InP clad layer 3b, an AlGaInAs distorted quantum well active layer 4b and a p-type InP clad layer 5b are formed like a mesa and a ridge section 6 is made up of these layers. A p-type InP buried layer 7, an n-type InP buried layer 8 and an buried layer 9 made of semi-insulating Fe-doped InP are buried on both sides of the ridge section 6, and an n-type InP buried layer 19 is further formed thereon. A current block layer 10 is made up of these layers.
A p-type InP layer 11 and a p-type InGaAs contact layer 12 are provided on the p-type InP clad layer 5b and n-type InP buried layer 19. An SiO2 film pattern 13 and a p-type electrode 14 are further formed thereon. An n-type electrode 15 is provided on the back of the InP substrate 2.
Next, the method of manufacturing the semiconductor laser shown in FIG. 17 will be explained. First, as shown in FIG. 18, crystals of an n-type InP layer 3c, an AlGaInAs distorted quantum well active layer 4c and a p-type InP layer 5c are made to grow one by one on the n-type InP substrate 2 using an metal organic chemical vapor deposition (MOCVD method).
Next, an SiO2 film (silicon oxide film) is formed on the p-type InP layer 5c shown in FIG. 18 and patterned. As a result, an SiO2 film pattern 16 is formed on the p-type InP layer 5c as shown in FIG. 19.
Next, the p-type InP layer 5c, AlGaInAs distorted quantum well active layer 4c, n-type InP layer 3c and n-type InP substrate 2 are wet etched using the SiO2 film pattern 16 shown in FIG. 19 as a mask. As a result, the ridge section 6 consisting of the n-type InP layer 3b, AlGaInAs distorted quantum well active layer 4b and p-type InP layer 5b is formed as shown in FIG. 20.
Next, the p-type InP buried layer 7, n-type InP buried layer 8, semi-insulating Fe-doped InP buried layer 9 and n-type InP buried layer 19 are formed one by one on both sides of the ridge section 6 shown in FIG. 20 using the MOCVD method. As a result, the buried current block layer 10 is formed as shown in FIG. 21.
Next, the SiO2 film pattern 16 shown in FIG. 21 is removed by etching. As a result, the structure shown in FIG. 22 is obtained. Next, as shown in FIG. 23, the p-type InP layer 11 and the p-type InP contact layer 12 are formed on then-type InP buried layer 19 and p-type InP layer 5b using the MOCVD method.
Next, the SiO2 film pattern 13 is formed on the p-type InP contact layer 12 shown in FIG. 23 and the p-type electrode 14 is formed thereon. Furthermore, the n-type electrode 15 is formed on the back of the InP substrate 2. As a result, the semiconductor laser shown in FIG. 17 can be obtained.
In the structure of the semiconductor laser shown in FIG. 17, the semi-insulating Fe-doped InP buried layer 9 blocks a current by trapping electrons. For this purpose, the top face and bottom face of the semi-insulating Fe-doped InP buried layer 9 need to be covered with the n-type InP layer.
However, in the process of forming the p-type InP buried layer 7, n-type InP buried layer 8, semi-insulating Fe-doped InP buried layer 9 and n-type InP buried layer 19 shown in FIG. 21, an MO (metal organic) gas hardly wraps around into the back side at both ends of the SiO2 film pattern 16.
Therefore, as shown in FIG. 24, at the top ends (the parts surrounded by dotted lines 20) on both sides of the ridge section 6, the top face of the semi-insulating Fe-doped InP buried layer 9 contacts the p-type InP layer 11. Therefore, the above described contact parts become a path through which a leakage current flows. That is, the above described conventional buried type semiconductor laser has a problem that the current-light output characteristic of the laser deteriorates due to the above described leakage current.